Estimating health parameters or symptoms of a degrading system

ABSTRACT

The present invention is concerned with the estimation of health parameters p(k) representing symptoms of a slowly degrading system, in particular industrial gas turbines. According to the invention, an estimation of a true health or independent parameter vector at time step k uses the estimation of the true health or parameter vector at a previous time step k−1 as a starting value for the production of a predicted health parameter vector at time step k. Based on the latter and a set of measured values of input variables of an extended model of the system, a prediction of output variables of the model is produced. This predicted model output is compared with measured values of the output variables to yield an error. From this error, a health parameter estimator in turn produces a health parameter estimate as a revision of the predicted health parameters. Preferably, the estimation of a true health or independent parameter vector at time step k includes a generalized process noise representing stochastic uncertainty and/or a-priori knowledge of an evolution of the health parameters.

FIELD OF THE INVENTION

The invention relates to the field of control and instrumentationtechnology for turbomachines such as gas turbines or internal combustionengines. It departs from a method of determining independent parametersrepresenting health states of a degrading system for the purpose ofdiagnosing a performance change as described in the preamble of claim 1.

BACKGROUND OF THE INVENTION

The goal of gas turbine performance diagnosis is to accurately detect,isolate and assess performance changes, system malfunctions andinstrumentation problems. Among a number of other techniques, Gas PathAnalysis (GPA) is a well established framework for estimating shifts inperformance from the knowledge of measured parameters, such as power,engine speeds, temperatures, pressures or fuel flow, taken along the gaspath of the turbine. Discernable shifts in these measured parametersprovide the requisite information for determining the underlying shiftin engine operation from a presumed reference, nominal or initial state,i.e. the degradation symptoms. GPA allows engine performancedeterioration to be identified in terms of a degradation of independentparameters or system states such as thermodynamic efficiencies, flowcapacities and inlet/outlet filter areas. In a subsequent diagnosisstep, these degradation symptoms can be analysed, from which amaintenance action schedule to ensure economic and safe operation isdeduced, or from which the remaining life of the major components ispredicted. The origin of a fault affecting a given component of the gasturbine can be of various nature, such as, for example, a contaminationof compressor blades, erosion of turbine blades or corrosion of machineparts. Conversely, different faults often create similar observableeffects or degradation symptoms.

According to the patent application EP-A 1 233 165, a degradation stateof an industrial gas turbine is determined with the aid of measurementsduring operation of the gas turbine. The proposed Gas Path Analysis usesa mathematical simulation or an analytical performance model of a gasturbine engine based upon component characteristics of the engine inquestion, such as compressor and turbine maps and includingthermodynamic and aerodynamic operating behaviour of the gas turbinesuch as the laws of conservation of energy and mass. The model permitsvalues of measurable output variables or dependent variables to bedetermined from input variables such as, for example, air inlettemperature and pressure, as well as from assumptions concerning stateor independent parameters corresponding to non-measurable degradationsymptoms. The output variables are, for example, pressures, mass flowsand temperatures at various points in the gas path of the gas turbine, afuel mass flow, a rotational speed and an output mechanical power.Deviations of state parameters, such as efficiencies or flow capacities,from a reference value represent symptoms of a degradation of acomponent of the gas turbine.

In particular, a deviation of a measured deteriorated performance y′from an iterated base-line performance is multiplied with a faultcoefficient matrix, derived from a theoretical relationship between theindependent parameters x and the dependent parameters y of the formy=F(x), to yield an improved estimation of the exact solution x′=F⁻¹(y′)and the next iterated base-line. In other words, a repeated applicationof linear Gas Path Analysis based on iterated base-line performance viathe Newton-Raphson technique is used to approach the exact solution,i.e. the independent vector x′ corresponding to the measureddeteriorated performance y′.

In the patent application EP-A 1 418 481 a framework for aero gasturbine diagnosis is proposed which distinguishes between rapiddeterioration due to singular system fault events and gradualdeterioration due to damage accumulation of all engine components. Ameasurement Δ vector, comprising deviations from a reference of some gaspath parameter data such as rotor speed, temperatures and pressures,reflects the effects of a multitude of possible engine/sensor systemfault occurrences as well as random measurement noise. From thismeasurement Δ vector at a discrete time k, a total fault vector x_(k)comprising the engine system and sensor faults as the current states isestimated within a Module Performance Analysis (MPA). Apart from areference to statistical tests or neural networks, the estimation methodis not detailed. If a rapid deterioration event is in progress, singlefault or root cause isolation is performed, based on the change ΔΔ_(k)in the measurement Δ vector w. r. t. the previous measurement at timek−1. Otherwise, multiple fault isolation is performed to yield anupdated error vector, based on the cumulative share Z_(k) ^(MFI) of themeasurement Δ vector assigned to gradual deterioration.

The different components of a gas turbine (GT), which consist mainly ofthe inlet nozzle, the compressor, the combustion chamber, the turbine,the cooling flow, and the outlet, all contribute—to a differentextent—to the degradation of GT performance. Because a small deviationfrom new-and-clean conditions already results in a significant loss ofperformance, the problem of identifying and localizing symptoms of theoverall degradation is of crucial importance.

DESCRIPTION OF THE INVENTION

It is therefore an objective of the invention to monitor the evolutionof different degradation symptoms of a slowly degrading system. Thisobjective is achieved by a method of estimating and a method ofsimulating health parameters representing symptoms of a slowly degradingreal or simulated system according to claims 1 and 9, respectively.Further preferred embodiments are evident from the dependent patentclaims.

According to the invention, an approximation of a true health orindependent parameter vector at time step k uses the approximation ofthe true health or parameter vector at a previous time step k−1 as astarting value for the production of a predicted or simulated healthparameter vector at time step k.

Based on the predicted health parameters and a set of measured values ofinput variables of an extended model of the system, a prediction ofoutput variables of the model is produced. This predicted model outputis compared with measured values of the output variables to yield anerror. From this error, a health parameter estimator in turn produces ahealth parameter estimate as a revision of the predicted healthparameters.

The approximation of a true health or independent parameter vector attime step k includes a generalised process noise representing stochasticuncertainty and/or a-priori knowledge of an evolution of the healthparameters.

Preferentially, the slowly degrading system is an industrial gas turbinethat operates in a steady state for long periods of time.Correspondingly, with the exception of some rare transient phasesbetween two distinct operating points, the temperature sensors are inthermodynamic equilibrium with the gaseous media and produce trustfulmeasurement values. In addition, the average update interval between twosuccessive time steps can be chosen quite long without approaching thecharacteristic degradation times of the turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail inthe following text with reference to preferred exemplary embodimentswhich are illustrated in the attached drawings, in which:

FIG. 1 schematically shows the components of a gas turbine,

FIG. 2 is a flow chart of the process of updating a health parameter,

FIG. 3 depicts the process of updating a health parameter by means of aKalman Filter,

FIG. 4 shows the on-line application of the above process,

FIG. 5 shows the off-line application of the above process,

FIG. 6 depicts a simulation approach based on a dynamic evolution model,and

FIG. 7 shows three graphs that chart the results of a simulation.

The reference symbols used in the drawings, and their meanings, arelisted in summary form in the list of reference symbols. In principle,identical parts are provided with the same reference symbols in thefigures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Albeit it is apparent to those skilled in the art that the presentinvention can be beneficially applied to a multitude of slowly degradingsystems such as all kinds of turbomachines and internal combustionengines subject to ageing, frictional wear, contamination by deposits orthe like, the following detailed description will concentrate onindustrial gas turbines. Industrial gas turbines are particularly suitedfor the proposed estimation of health parameters because, unlike e.g.aero gas turbines, they are immobile and operate in a steady state forlong periods of time. Therefore, neither abruptly changing ambientparameters as experienced by moving turbines, nor imbalanced exhaustparameters following an acceleration phase, are generally to be expectedin the case of industrial gas turbines.

FIG. 1 shows the principal components of a gas turbine and the placeswhere the various input and output variables are measured. The gasturbine is viewed as a system which consists of a sequence of idealvolume elements in thermodynamic equilibrium, i.e. compressor inlet 2 a(filter, nozzle), compressor 2 b, combustion chamber 2 c, turbine 2 dand outlet conduit 2 e, wherein compressor 2 b and turbine 2 d aremechanically interconnected by a shaft 2 f. A flow w_(a) of ambient airat temperature T₀, pressure p₀ and relative humidity H₀ enters thecompressor inlet 2 a through inlet guide vanes characterized by thevariable angle α_(VIGV). Compressor inlet and outlet temperature andpressure are denoted T₁, p₁ and T₂, p₂, respectively. A fuel mass floww_(f) of temperature T_(f) and fuel heat value Δh_(f), as well as awater mass flow w_(w) are injected to the combustion chamber 2 c. Aturbine inlet gas flow w_(g) enters the turbine 2 d at a turbine inletpressure p₃ and temperature T₃. Turbine outlet pressure and temperatureare denoted p₄ and T₄, whereas the exhaust gas finally has a temperatureT₅ and a pressure p₅ assumed equal to p₀. The shaft speed N, mechanicalpower P_(mech) and generator power P_(gen) are typical output variables,whereas the ambient parameters T₀, p₀, H₀ and the regulated quantitiesw_(f), w_(w), α_(VIGV) are typical input variables.

System models of gas turbines replicate the thermodynamic andaerodynamic operating behaviour of the gas turbine and include the lawsof conservation of energy and mass, enthalpy considerations as well asturbine and compressor maps provided by the manufacturer. The health ordegradation of a compressor or a turbine is usually described by apolytropic efficiency η_(∞) a flow capacity m√T_(IN)/p_(IN), where m isthe flow and T_(IN) and p_(IN) are the inlet temperature and pressure ofthe device, respectively, and where the efficiencies actually may befactors multiplying a power-dependent reference, nominal or initialefficiency. Accordingly, common equipment faults of a gas turbine may berepresented by deviations of so-called health parameters from the valuesof a new-and-clean state. In what follows, an illustrative exampleincluding these four standard degradation symptoms or health parametersis considered, thereby disregarding e.g. the effective area of an inletfilter. The named four health parameters, i.e. compressor flow capacity,polytropic compressor efficiency, turbine flow capacity, and polytropicturbine efficiency are grouped as the elements of a vector p ofindependent parameters. As mentioned before, these four healthparameters cannot be measured directly during operation of the realplant; they are “hidden” symptoms that have to be derived orreconstructed from the overall plant measurements.

FIG. 2 depicts a flow chart of the model-based process of assessing thehealth parameter degradation according to the invention. As set outabove, the health states represented by the vector p of independentparameters are linked through a thermodynamic model of the gas turbineto input variables u (ambient conditions as well as regulated quantitiesor manipulated variables such as the angle of the variable inlet guidevanes and the fuel mass flow) and dependent output variables y (internalprocess variables such as temperatures, pressures, power, shaft speed).An estimation or approximate reconstruction of the true health orindependent parameter vector p(k) at time k uses the estimatep_(est)(k−1) of the true health or parameter vector p(k−1) at theprevious time step k−1 as a starting value. Based on this estimate, ahealth parameter predictor 11 produces a predicted health parameterp_(pred)(k) as detailed further below. An extended model 20 of thesystem yields a predicted model output y_(pred)(k) based on thepredicted health parameter p_(pred)(k) and a set of measured inputvalues u(k). The predicted model output y_(pred)(k) is compared with ameasured dependent vector y(k) in a comparator 12 to yield an errore(k). From the error e(k), a health parameter estimator 13 in turnproduces a health parameter estimate p_(est)(k) as a revision of thepredicted health parameter p_(pred)(k). This estimate p_(est)(k) isassociated with the unknown true value of the independent parametervector p(k) at time k. The time delay element 14 is stores the actualestimate p_(est)(k) and provides it to the predictor 11 as the previousestimate as soon as updated measured values are available at time k+1.

The proposed method is based on the assumption that an adequatelyadapted average update interval, i.e. a time delay D=t(k)−t(k−1) betweentwo successive executions being well below the characteristicdeterioration times τ, prevents excessive shifts in the parameters. Inthe case of gas turbines, the expected degradation rates of the healthparameters are of the order of weeks or months, such that a time delayof the order of minutes appears to be adequate. More frequent updatescould not be expected to yield any additional information, whereas withlonger update intervals the advantage of starting with the previousestimate p_(est)(k−1), instead of any “universal” baseline related to astandard or non-deteriorated state, would gradually disappear.

As indicated by the dashed line in FIG. 2, input variables u, and amongthese in particular the regulated quantities, may be taken into accountfor the prediction of the health parameters. Thus, the knowledge aboute.g. a changing angle of the variable inlet guide vanes can be includedalready when predicting the parameters rather than when revising theprediction via the measured output variables y being affected by theformer.

The tracking of the degradation symptoms, i.e. the repeatedidentification of the health parameters, is done via a Recursive LeastSquares algorithm (RLS), or preferably via a specific discrete-timeKalman Filter (KF). FIG. 3 represents a graphical view of the use of thespecific Kalman Filter, including

-   -   a dynamic evolution model 10 of the health degradation        parameters p, including the aforementioned predictor 11,        comparator 12, estimator 13 and delay element 14, and    -   an extended system model 20 including an inverse input sensor        model 21, a thermodynamic model of the gas turbine 22, and an        output sensor model 23 for predicting a vector y_(pred)(k) of        model outputs.

In the case of an industrial gas turbine, the thermodynamic model 22preferably describes the gas turbine in a steady-state phase, i.e.transient phases between two operating points or a frequency-responseoperation mode are not considered. The only dynamic aspect is theparameter evolution as described in the predictor 11.

As set out above, at each time step k, the estimate p_(est)(k−1)obtained at the previous execution is taken as a starting value for theprediction, which optionally includes the current values u(k) of theinput variables as indicated by the dashed line, of the predicted healthparameter p_(pred)(k). The latter in turn is fed into the thermodynamicmodel of the gas turbine 22 to yield a prediction of model outputsy_(pred)(k) corresponding e.g. to measurements of output variables alongthe gas path. The difference e(k) between these predicted model outputsy_(pred)(k) and the true measurements y(k) is combined with a KalmanGain K(k) by matrix-vector multiplication to yield the update termΔp(k)=K(k)·e(k). The update term Δp(k) finally is added as a revision tothe predicted parameter vector p_(pred)(k) to form the estimatep_(est)(k) of the health parameters p(k) as a reconstruction of thedegradation symptoms of the gas turbine.

In standard Kalman Filter terminology, the prediction of states p andmodel outputs y is done via two functions denoted f and h that can beassigned to the predictor 11 and the extended system model 20,respectively, and can be formulated asp _(pred)(k)=f(p _(est)(k−1), u(k))y _(pred)(k)=h(p _(pred)(k), u(k))

In a general way, the Kalman Filter takes uncertainties in the initialstate estimation, in the dynamic evolution of the states, and in themeasurements into account. In the context of the present invention, thehealth parameters take the role of the states, and the uncertaintyresults from offsets, drifts and stochastic effects of the physicalsensors that determine the values u(k), y(k) of the input and outputvariables. Likewise, the step of passing from the previous estimationp_(est)(k−1) to the current prediction p_(pred)(k) may comprise aninherent degree of uncertainty, which however the present invention isable to handle as detailed below. To this purpose, the input and outputvariables, as well as the health parameters, will be considered asvectors of distributed quantities with a mean vector and a covariancematrix from here on.

FIG. 3 depicts the background or motivation for including the sensornoise that inevitably accompanies the measured values of the input andoutput variables. The measured value u(k) as determined by a physicalinput-sensor is treated according to the input-sensor model 21 whichacts as an inverse sensor model. This model subtracts an assumedinput-sensor noise ξ_(u)(k), approximating offset, drift and stochasticcharacteristics of the input-sensor, from the measured value u(k) toyield a corresponding internal state. The latter is considerederror-free and fed into the thermodynamic model 22, which in turnprocesses exclusively mean values. Correspondingly, the resultinginternal state, likewise considered error-free, is treated according tothe output-sensor model 23, which adds an assumed output-sensor noiseξ_(y)(k) to yield the predicted model output y_(pred)(k). The latterthen is compared with the measured value y(k) as determined by aphysical output-sensor, the offset, drift and stochastic characteristicsof which is being approximated by the assumed output-sensor noiseξ_(y)(k). In Kalman Filter terminology, the negative input sensor noiseξ_(u)(k) refers to process noise, whereas the output sensor noiseξ_(y)(k) is known as measurement noise. Both process noise ξ_(u)(k) andmeasurement noise ξ_(y)(k) are each described by a mean vectorcomprising as many elements as there are input and output variables,respectively, and generally equaling zero. In addition, the respectivecovariance matrices Q and R even allow taking into account a mutualcorrelation between the respective sensor noise terms. In the KalmanFilter according to FIG. 3, process and measurement noise are the basisof the calculation of the Kalman Gain K(k) as part of the parameterevolution model 10.

At predictor 11, mean value and variance of the latest estimationp_(est)(k−1) are updated to handle any offset, drift and stochasticuncertainty connected to the step of passing from the previousestimation p_(est)(k−1) to the current prediction p_(pred)(k). To thatpurpose, a generalized process noise ξ_(p)(k), described by a meanvector with as many elements as there are health parameters, as well asby a corresponding covariance matrix Q_(pp), is added to the former.This generalized process noise is also included in the calculation ofthe Kalman Gain K(k). Using the covariance matrices mentioned, and inaddition to the estimation p_(est)(k) of the health parameters, theKalman Filter delivers as well the covariance matrix P_(pp)(k)indicative of the estimation error, i.e. describing the uncertainty inthe estimation p_(est)(k).

The covariance matrix Q_(pp) of the generalized process noise ξ_(p)(k)represents e.g. the increasing uncertainty resulting from a longer thanusual delay or update time D, its elements may typically vary between10⁻¹⁰ and 10⁻⁵ in relative units. A priori knowledge about the healthparameter changes, such as speed of condition degradation of aparticular gas turbine or the scheduling of maintenance actions, can beincluded through the specification of the statistics of ξ_(p)(k). As afinite degradation speed is reflected by a mean value of the generalizedprocess noise ξ_(p)(k) different from zero, a maintenance effect at timek can be modelled by a corresponding modification of the mean ofξ_(p)(k), or randomly occurring damages can be modelled by adding anoffset to ξ_(p)(k).

The proposed health parameter estimation setup may be advantageouslyused in the context of the following embodiments directed to an on-lineor off-line analysis for condition monitoring.

FIG. 4 shows the on-line approach, where the system continuously triesto identify the latest evolution of the different degradation symptoms.As new measurements u(k), y(k) are provided at time k, e.g. by adistributed control system 3, the dynamic evolution model 10 and theextended system model 20 are recomputed as detailed above. This providesa new estimate p_(est)(k) of the current value of the health parameters,which is stored in a database 4, used to update a trend display 5, orfed to a diagnosis tool 6 for the purpose of diagnosing an underlyingfault.

FIG. 5 shows the setup for an off-line or post-mortem analysis. A batchof historical system data comprising values u({k}), y({k}) of thevarious output and input parameters measured at a plurality of discretetime steps {k}, is stored in a database 4 and made available to theestimator 13. Given a reliable starting point in the form of an initialestimation p_(est)(0) and assumptions about sensor noise or drift, thesystem estimates a sequence p_(est)({k}) of health parameters, thusreconstructing the past evolution of the different degradation symptoms.

If appropriate assumptions about the evolution of the various factorsinfluencing on the generalized process noise ξ_(p)(k) are made, theproposed health parameter predictor 11 as part of the dynamic evolutionmodel can also be used for simulating a simultaneous degradation ofdifferent and possibly coupled health parameters. As above, the elementsof the generalized process noise ξ_(p)(k) describe the change of thecorresponding elements of the health parameter vector. Such a change orupdate is random and assumed independent of the current state andcomposed of various stochastic contributions 15 a, 15 b, 15 c asschematically depicted in FIG. 6. Degradation effects, which result in agradual decrease in efficiency and flow capacity are modelled by auniformly distributed drift. The mean of this first stochasticcontribution is preferably exponential in time. In a second stochasticcontribution, a maintenance schedule set up according to the operationalconstraints results in a recovery in efficiency and flow capacity. In athird stochastic contribution, randomly occurring damage effectsresulting in a step-like decrease in efficiency are modelled forinstance as a Poisson distribution. The generalized process noiseξ_(p)(k) is the sum of all these components, which determines theevolution of the vector of simulated parameters p_(sim)(k) as indicated.The arbitrary time delay element D denotes the time-discrete characterof this process that as a matter of fact corresponds to ashort-circuited estimator 13, where the estimated values are identifiedwith the predicted values without any revision at all.

FIG. 7 finally shows the details of such a simulation over a time periodof 20 days. The topmost graph depicts, in relative units, two discretemaintenance effects 15 b after 6 and 13 days as well as a singulardamage event 15 c (dashed line) after 16 days. On the second graph, ageneralized process noise ξ_(p)(k) is displayed, resulting from theaforementioned contributions 15 b, 15 c as well as a continuousdegradation modelled by a uniformly distributed drift and simulated onceper hour. The third graph shows the evolution of simulated parametersp_(sim)(k).

This procedure of simulating the health parameters can further beextended to simulate other aspects of the system behaviour. In atraining environment, the simulated parameters p_(sim)(k) are fed intothe system model 22 together with a batch of historical system data ofmeasured input values u({k}) to produce simulated output valuesy_(sim)({k}). The latter are then compared with measured output values,and the initial health parameter simulation, and the assumptions aboutthe generalized process noise ξ_(p)(k) in particular, can be tuned ortrained. In a test schedule environment, the parameters p_(sim)(k)simulated according to a particular schedule of maintenance decisions,together with a batch of input values u({k}), are used to simulate asystem performance or another element of the output vector y.Accordingly, a plurality of health parameter degradation scenarios canbe produced and evaluated for comparative purposes.

While the preferred embodiments of the present invention have beenillustrated and described, it will be clear that the present inventionis not limited to these embodiments only. Numerous modifications,changes, variations, substitutions and equivalents will be apparent tothose skilled in the art without departing from the spirit and scope ofthe present invention as described in the claims.

List of Designations

-   2 a inlet-   2 b compressor-   2 c combustion chamber-   2 d turbine-   2 e outlet conduit-   2 f shaft-   10 dynamic evolution model-   11 health parameter predictor-   12 comparator-   13 health parameter estimator-   14 delay element-   15 stochastic contribution-   20 extended system model-   21 input sensor model-   22 thermodynamic model of the gas turbine-   23 output sensor model-   3 distributed control system-   4 database-   5 trend display-   6 diagnosis tool

1. A method of estimating health parameters p(k) representing symptomsof a slowly degrading system that includes at least turbomachines andinternal combustion engines at a discrete time k, the method beginexecuted in a processing system having a specific discrete-time Kalmanfilter having a dynamic evolution model and an extended system model,the method comprising a) providing, in the Kalman filter, a predictionPpred(l) of the health parameters p(k), b) calculating, in the Kalmanfilter, a prediction Ypred(k) of output variables y of a model of thesystem, based on a prediction Ppred(k) of the health parameters andmeasurements u(k) of input variables u of the model measured at time k,c) establishing, in the Kalman filter, a difference e(k) between theprediction Yprep(k) of the output variables y and measurements y(k) ofthe output variables y measured at time k, d) calculating, in the Kalmanfilter, an estimate Pest(k) of the health parameters p(kl) based on theprediction Ppred(k) of health parameters P(k) and the difference e(k),wherein step a) comprises providing a prediction Ppred(k) of healthparameters p(k) based on an estimate Pest (k−1) of the health parametersp(k−1) at a previous time k−1, wherein an interval D between the time kand the previous time k−1 is smaller than a characteristic degradationtime τ of the degrading system, and e) simulating a system performancebased on the prediction P_(pred)(k) of the health parameters p(k) toproduce simulated output values ysim({k}).
 2. The method according toclaim 1, wherein step a) comprises providing the prediction Ppred(k)based on generalized process noise ξ(k) representing stochasticuncertainty and/or a-priori knowledge of an evolution of the healthparameters p(k).
 3. The method according to claim 1, wherein step a)comprises providing the prediction Ppred(k) based on the measurementsu(k) of the input variables u measured at time k.
 4. The methodaccording to claim 1, wherein step b) comprises calculating a predictionYpred(k) of output variables y based on output-sensor noise ξy(k)representing offset, drift and/or stochastic characteristics of anoutput-sensor providing the measurements y(k) of the output variables yat time k.
 5. The method according to claim 2, wherein steps a) to d)are performed by a specific discrete-time Kalman Filter comprising adynamic evolution model and an extended system model, and including aKalman Gain K(k) based on the generalized process noise ξ(k) or theoutput-sensor noise ξy(k).
 6. The method according to claim 1, whereinthe method is used for estimating health parameters p(k) representingsymptoms of an industrial gas turbine.
 7. The method according to claim1, wherein the steps a) to d) are executed intermittently after thelapse of an update interval D to produce a current estimate Pest(k) ofthe current value of the health parameters p(k) based on currentmeasurements u(k), y(k) of the input and output variables u, y, orrepeatedly to produce a series of estimates (Pesk{k} based on historicalbatch data comprising values u({k}), y({k}) of the input and outputvariables u, y measured at a plurality of discrete time steps {k}.
 8. Acomputer readable medium that stores a computer program for estimatingor simulating health parameters p(k) representing symptoms of a slowlydegrading system at a discrete time k, the computer program comprisingcomputer program code means to make, when the computer program is loadedin an internal memory of a digital computer, said computer beingconfigured to execute a method of estimating health parameters p(k)representing symptoms of a slowly degrading system at a discrete time k,the method begin executed in a processing system having a specificdiscrete-time Kalman filter having a dynamic evolution model and anextended system model, the method comprising a) providing, in the Kalmanfilter, a prediction Ppred(l) of the health parameters p(k), b)calculating, in the Kalman filter, a prediction Ypred(k) of outputvariables y of a model of the system, based on a prediction Ppred(k) ofthe health parameters and measurements u(k) of input variables u of themodel measured at time k, c) establishing, in the Kalman filter, adifference e(k) between the prediction Yprep(k) of the output variablesy and measurements y(k) of the output variables y measured at time k, d)calculating, in the Kalman filter, an estimate Pest(k) of the healthparameters p(k1) based on the prediction Ppred(k) of health parametersP(k) and the difference e(k), wherein step a) comprises providing aprediction Ppred(k) of health parameters p(k) based on an estimatePest(k−1) of the health parameters p(k−1) at a previous time k−1,wherein an interval D between the time k and the previous time k−1 issmaller than a characteristic degradation time τ of the degradingsystem, and e) simulating a system performance based on the predictionP_(pred)(k) of the health parameters p(k) to produce simulated outputvalues ysim({k}).
 9. The method according to claim 4, wherein steps a)to d) are performed by a Kalman Filter comprising a dynamic evolutionmodel and an extended system model, and including a Kalman Gain K(k)based on the generalized process noise (k) or the output-sensor noisey(k).
 10. A health parameter estimator for estimating health parametersp(k) representing symptoms of a degrading system at a discrete time k,comprising a) a health parameter predictor providing a predictionP_(pred)(k) of the health parameters p(k), b) an extended system modelfor calculating a prediction Y_(pred)(k) of output variables y of themodel, based on the prediction P_(pred)(k) of the health parameters andmeasurements u(k) of input variables u of the model measured at time k,c) a comparator for establishing a difference e(k) between theprediction y_(pred)(k) of the output variables y and measurements y(k)of the output variables y measured at time k, d) a health parameterestimator for calculating an estimate P_(est)(k) of the healthparameters p1(k) based on the prediction P_(pred)(k) of the healthparameters p(k) and the difference e(k), wherein the health parameterpredictor is adapted to provide a prediction P_(pred)(k) of the healthparameters p(k) based on an estimate P_(est)(k−1) of the healthparameters p(k−1) at a previous time k−1, wherein an interval D betweenthe time k and the previous time k−1 is smaller than a characteristicdegradation time τ of the degrading system and wherein components a) tod) utilize a specific discrete-time Kalman Filter having a dynamicevolution model and an extended system model, and a health parametersimulator that simulates a system performance based on the predictionp_(pred)(k) of the health parameters p(k) to produce simulated outputvalues ysim({k}).
 11. The health parameter estimator according to claim7, characterized in that it comprises a specific discrete-time KalmanFilter comprising the extended system model and a dynamic evolutionmodel including said predictor, comparator, estimator as well as a delayelement, and comprising a Kalman gain K(k) based on a generalizedprocess noise ξ (k) representing stochastic uncertainty and/or a prioriknowledge of an evolution of the health parameters p(k) or based onoutput-sensor providing the measurements y(k) of the output variables yat time k.